In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, introduction of a dual connectivity scheme (Dual connectivity) in Release 12 and thereafter is planned (see Non Patent Document 1). In the dual connectivity scheme, a mobile station receives user data from a plurality of base stations. To the mobile station, a radio resource is assigned from each base station, and thus, it is possible to expect an improvement in throughput.
The dual connectivity scheme is considered on the assumption that it is applied to transmission and reception of downlink data, and of a plurality of base stations that establish a connection with a mobile station, one base station (hereinafter, referred to as “master base station”) only establishes an RRC connection with the mobile station. On the other hand, of the plurality of base stations, another base station (hereinafter, called “secondary base station”) provides an additional radio resource to the mobile station without establishing an RRC connection with the mobile station. That is, it is considered on the assumption that the master base station provides all the functions necessary for the communication between the master base station and the mobile station, and the secondary base station provides only a function necessary for transmitting the user data. It is noted that the dual connectivity scheme may also be called an inter-base station carrier aggregation (inter-eNB CA).